Abstract: R2.00005 : Spin-orbit coupling in organic spintronics*

Author:

Zhi-Gang Yu(SRI International)

I will talk about spin-orbit coupling (SOC) in
$\pi$-conjugated organicmaterials and its effects on
spin characteristics including the spin-relaxation time, spin-diffusion length, and $g$ factor [1].
While $\pi$ electrons are
responsible for low-energy electrical and optical processes in $\pi$-conjugated organic solids, $\sigma$ electrons must be
explicitly included to properly describe the SOC. The SOC mixes up- and down-spin states and, in the context of
spintronics, can be quantified by an admixture parameter in the electron and hole polaron states in $\pi$-conjugated
organics. Molecular geometry fluctuations such as ring torsion, which are common in soft organic materials and
may depend on sample preparation, are found to have a strong effect on the spin mixing. The SOC-induced
spin mixing leads to spin flips as polarons hop from one molecule to another, giving rise to spin relaxation
and diffusion. The
spin-relaxation rate is found to be proportional to the carrier hopping rate. The
spin-diffusion length depends on the spin mixing and hopping distance but is insensitive to the carrier mobility.
The SOC influences the $g$ factor of the polaron state and makes it deviate from the
free-electron value.
The SOC strengths in common organics are quantified based on first-principles calculations and their values
in tris-(8-hydroxyquinoline) aluminum (Alq$_3$) and in copper phthalocyanine (CuPc) are
particularly strong, due to the orthogonal arrangement of the three ligands in the former and Cu $3d$ orbitals in
the latter. The theory quantitatively explains the recent measured spin-diffusion lengths in Alq$_3$ from muon spin
rotation and in CuPc from spin-polarized two-photon photoemission.
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[1] Z. G. Yu, Phys. Rev. Lett. {\bf 106}, 106602 (2011); Phys. Rev. B {\bf 85}, 115201 (2012).

*This work was supported by ARO, DOE, and SRI internal funds.

To cite this abstract, use the following reference: http://meetings.aps.org/link/BAPS.2013.MAR.R2.5